Shale oil recovery process using heated oil-miscible fluids



M. PRATS June 2, 1970 SHALE OIL RECOVERY PROCESS. USING HEATED OIL-MISCIBLE FLUIDS Filed April 19, 1967 FIG. 2

FIG.4

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INVENTORI MICHAEL BYI 9F'ATS HIS ATTORNEY FIG United States Patent Olce 3,515,213 Patented June 2, 1970 3,515,213 SHALE OIL RECOVERY PROCESS USING HEATED OIL-MISCIBLE FLUIDS Michael Prats, Houston, Tex., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Apr. 19, 1967, Ser. No. 632,006 Int. Cl. E21b 43/22, 43/24 U.S. Cl. 166-252 5 Claims ABSTRACT OF THE DISCLOSURE Shale oil is recovered from a subterranean oil shale formation by circulating a fluid heated at a moderate temperature from one point within the formation to another for a relatively long period of time until a significant proportion of the organic components contained in the oil shale formation is converted to oil-shale-derived lluidizable materials.

BACKGROUND OF THE INVENTION The invention relates to a method for recovering shale oil from an oil Shale formation. More particularly, it relates to a method of recovering shale oil from an oil shale formation by heating the walls of a permeable channel to a moderate temperature for a relatively long period of time, thereby recovering kerogen components that are being converted to lluidizable materials while the zone that is being heated expands.

The kerogen in an oil shale formation can be converted to fluidizable materials by prior art in-situ retorting processes. In these prior art processes, temperatures of over 700 F., and preferably 950 F., however, are used to convert the solid organic matter in the oil shale to useful recoverable products in relatively short times.

These prior art processes generally involve drilling wells into the oil shale formation and creating a horizontal fracture between the Wells to -provide a path for injected and produced fluids. A burner is operated in the injection well to start combustion in the oil shale formation. The burning zone then is moved outward from the injection Well at a controlled rate while heat flows by conduction from the burning zone to adjacent oil shale. The hydrocarbons produced -by pyrolysis llow into the stream of -gaseous combustion products and are swept through the fracture system into producing wells. Very complex heat transfer problems are involved. Relatively high temperatures, for example, 900-95 0 F., are required for etllcient separation of the hydrocarbons from the oil shale.

SUMMARY OF THE INVENTION It has been found that the kerogen in an oil shale formation undergoes a low-temperature conversion to fluidizable materials capable of being displaced, dissolved, or entrained in a hot fluid that contacts the oil shale. It is becoming increasingly evident that it may prove to lbe economically attractive to produce shale oil from an oil shale formation by a relatively long, moderately low temperature heating operation. f

In accordance with the teachings of the invention, the kerogen in an oil shale formation, which is normally a substantially insoluble solid material, is converted to fluidizable materials at a slow but significant rate at temperatures that are well below the usual retorting temperatures, the latter being generally above 900 F. Thus, the retorting or pyrolyzing conversions of the components of oil shales are not reactions in which a relatively high threshold temperature must be exceeded before a conversion occurs.

In general, the invention consists of a process for producing hydrocarbons from a subterranean oil shale formation by extending at least one well borehole into a subterranean oil shale formation, determining the depths of at least two points that encounter a zone that is capable of forming a permeable channel within the oil shale formation wherein at least one of the points is encountered by at least one of the boreholes, establishing preferential lluid communication between at least one of the boreholes and the adjacent oil shale at both of the so-encountered points and forming a permeable channel that extends through the oil shale formation from one to another of the points. Fluid is then circulated through the permeable channel from one to another of the points while heating and flowing the fluid at rates such that the temperature of the outgoing iluid is from about 10 F. greater than the natural temperature of the oil shale to about 600 F. until at least some of the organic components of the oil shale are converted to oil shale derived lluidizable materials capable of being extracted from the oil shale by entraining them in the circulating fluid.

It is an object of this invention to utilize the practices discussed above to recover oil from a subterranean oil shale formation while circulating lluid through the oil shale at much lower temperatures than required previously.

Other objects of this invention will be pointed out in the following detailed description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principles of this invention and the preferred method of applying these principles.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a vertical view of an oil shale stratum to which the recovery process of this invention has been applied involving a single well.

FIG. 2 is a vertical sectional view of an oil shale stratum to which the recovery process of this invention has been applied to a pair of wells.

FIG. 3 is a vertical sectional view of an alternate recovery process of the invention applied to a single well.

FIG. 4 is a graphical illustration of the weight loss of an oil shale formation over a relatively long period of time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning to the drawing, FIG. l shows awell borehole 11 extending into subterranean oil shale formation 12. Fluid communication is established between points 13 and 14 in oil shale formation 12 and adjacent to borehole 11 along a vertical fracture by', for example, a conventional hydraulic fracturing procedure. A iluid is then circulated through tubing 17 past packers 18 and 19', while being heated, until the oil shale derived lluidiza'ble materials are entrained in the circulating fluid. 'Ihe fluid passes through perforations 20 and 21 in casing 22. Of course, if the Wellbore 11 is uncased, such perforations would be unnecessary. These iluidizable materials can then be recovered from the outilowing portions of the circulating fluid by any known means. Thus, a single well may be used, although it is generally preferred to use at least a pair of wells. As seen in FIG. 1, if a single well is used, the preferred two points may be a pair of substantially vertically separated points that are apt to be encountered by vertical fractures within the oil shale.

As shown in FIG. 2, a pair of wellbores 23 and 24 extend into subterranean oil shale formation 25. Fluid communication is established between point 26 adjacent wellbore 23 and point 27 adjacent wellbore 24. In a preferred embodiment, the depths of such points may be those at which a tuaceous streak is encountered by a pair of wells between which the streak is continuous. The permeable channel extending through the oil shale may be formed by the process of locating and acidizing a tuffaceous streak, described in application Ser. No. 619,259 filed Feb. 28, 1967. A fluid is then circulated through tubing 25 past packer 30, while being heated, until the oil shale derived fluidizable materials are entrained in the circulating fluid. The circulating fluid would pass through perforations 31 in the casing 32 of wellbore 23, through points 26 and 27, and through perforations 33 in the casing 34 of wellbore 24. Again, if the Well is uncased, such perforations would be unnecessary. Fluidizable materials which are derived from the oil shale can then be recovered from the circulated fluid by any known means.

The circulation of the hot fluid may be a long-duration heating operation and, for some time, the amount of oil production may be insignificant. The temperature of the circulating fluid is preferably monitored either at the point at which the fluid flows out of the permeable path or the wellhead.

Oil shales are generally impermeable. Once a permeable path has been established between a pair of Wells, the permeable path will provide substantially the only zone that can be penetrated by a fluid injected into either of the wells. In view of this, relatively simple equipment can be utilized to circulate the heated fluid through the permeable path between the selected points. The fluid can be pumped through a heating device, through the permeable path, through a temperature-monitoring device, and then recycled back through the heating device. The duration of the heating that is necessary for a given oil shale can be determined by maintaining a sample of the shale at an equivalent temperature for an equivalent time until a suitable degree of conversion is obtained. This can be done prior to or while circulating the fluid.

In FIG. 3, an alternate recovery process, which can be operated with a single well, is illustrated. Here, the permeable channel formed within oil shale formation 12 is preferably a relatively voluminous permeable fragmented zone 35. The term permeable fragmented zone refers to a multiply fractured zone in Which the number of the fractures and the volume of the interconnected openings Within the fractures provide a void volume of from about to 40 percent of the volume of the zone.

Permeable fragmented zones can be formed by known hydraulic and/or explosive techniques for fracturing subsurface earth formations. One suitable fracturing technique was described in 1922, in Pat. 1,422,204. The streak acidizing procedure of application Ser. No. 619,259, filed Feb. 28, 1967 can be used, preferably to form a channel into which a liquid explosive is injected and subsequently detonated to form a generally disc-shaped permeable fragmented zone. High-power explosives, such asthose produced by nuclear devices, are particularly suitable means for forming such fragmented zones. In general, the permeable fragmented zone formed by a nuclear de- 'vice has a vertically extensive and generally cylindrical shape. i

In circulating heated fluid through a permeable fragmented zone, the flow paths can be vertical or horizontal and can involve a radially-expanding or line-drive type displacement of the fluid that is circulated through the oil shale. Generally, a substantially vertical downward flow is preferred.

FIG. 3 illustrates a portion of a nuclear chimney type of permeable fragmented zone 35. In treating such a zone, one or more wells 36 are drilled to near the bottom, preferably while the zone is hot, or at least warm, from the explosion energy. In the illustrated arrangement of FIG. 3, the Well 36 is drilled and cased to near the bottom and the casing 37 is perforated at 38 and 39 and equipped for injecting uid through the borehole annulus above packer 18, and through perforations 38 into the upper portion of the fragmented zone. Fluid is produced 4 from near the bottom of the zone through perforations 39 and tubing string 40.

With such an arrangement, the pressure within the permeable fragmented zone is adjusted to one selected for the circulation of heated fluid. The adjustment is affected by controlling the rate of withdrawing fluid from the cavern relative to the rate of injecting fluid into the cavern. As indicated in FIG. 3, conventional equipment and techniques, such as heater 41, pump 41a, separator 42 and heat exchanger 43, can be used for pressurizing, heating, injecting, producing, and separating components of the fluid that are circulated through the permeable zone 35. The production of the fluid can be aided by downhole pumping means, not shown, or restricted to the extent necessary to maintain the selected pressure within the zone. The pressure in the zone is preferably maintained at a level suited for economically transferring heat into the zone by circulating a fluid that is economically available at the Well site.

A Wide variety of fluids can be used in this process. The main requirements are that the fluid be pumpable at a moderate temperature such as from about 400l to 600 F. Aqueous liquids or streams of various grades, such as low quality steam, dry steam, or super saturated steam, can be used. Such aqueous fluids should be softened as required to inhibit scaling at the temperatures to which they are heated. Oil-miscible fluids are generally preferred. The relatively low cost volatile hydrocarbons that contain or consist essentially of volatile oil shale hydrocarbons are particularly suitable.

In certain situations, it is advantageous to circulate a mixture of relatively low molecular weight, predominantly aromatic hydrocarbons having relatively low critical temperatures and pressures. With such hydrocarbons (which may include significant proportions of shale oil hydrocarbon) the temperatures and pressures Within the permeable zone can provide conditions approaching or exceeding the critical conditions for part or all of the circulating hydrocarbons. In the critical or supercritical region, such hydrocarbons have densities and viscosities that are intermediate between their gas and liquid states and are particularly effective in extracting organic components from oil shale.

FIG. 4 shows graphically the heating of oil shale over a period of days at a temperature of 550 F. and 60 p.s.i.g., total pressure. The results were plotted to show the percent weight loss per time being heated. These results show that fluidization occurs at a relatively slow, but significant rate.

It has been found that the pyrolysis of the oil shale utilizing the process of the instant invention is significantly benefited by the presence of hydrogen sulfide. Thus, in this process it is desirable, in certain situations, to mix significant proportions of hydrogen sulfide or hydrogen sulfide plus hydrogen with either or both the fluid which is circulated to effect the heating and the fluid which is circulated to displace or extract the oil. Such situations would be those in which an adequate supply of such additives are available at the well site at relatively lowcost. Such additives are preferably used, with recycling, where an oil miscible uid is being circulated, separated from the entrained oil and recycled. Significant proportions would be those capable of causing a significant increase in the rate of oil production. Such proportions may be as low as about one mole percent of the circulating fluid. These proportions can be increased to whatever extent is economically desirable.

I claim as my invention:

1. In a process for producing hydrocarbons from a subterranean oil shale formation comprising the steps of:

extending at least one well borehole into a subterranean oil shale formation;

determining the depth of at least tw'o points at which a zone capable of forming a permeable channel Within the oil shale formation is adjacent to at least one well borehole that extends into the oil shale formation;

establishing ud communication between at least one of said boreholes and the adjacent zone at at least two of said points and forming a permeable channel that extends through the oil shale from one to another of said points; and

circulating an oil-miscible fluid containing hydrogen sulfide therein through the permeable channel from one to another of said points while heating and flowing the uid at rates such that the temperature of the outgoing uid is from about 10 F. greater than the natural temperature of the oil shale to about 600 F. until a significant proportion of the organic components of the heated oil shale are converted to oil-shale-derived uidizable materials.

2. In the process of claim 1 including the step of extracting the oil-shale derived fiuidizable materials from the heated oil shale by entraining such materials in outowing portions of the circulating fluid.

3. In the process of claim 2 including the step of recovering the oil-Shale-derived uidizable materials from the outowing portions of the circulating uid.

References Cited] UNITED STATES PATENTS Marx et al. 166-11 Graham 166-11 X Thomas 16611 X Strubhar 166-11 Jacobs et al. 166-11 Vogel 166-11 X Needham 166-11 STEPHEN J. NOVOSAD, Primary Examiner U.S. Cl. X.R. 

